Grooving on Phobos

Phobos, the larger moon of Mars, has a surface covered in craters, dust, boulders – and a great many semi-parallel and intersecting grooves. One theory for the grooves’ origin, proposed in 2011, holds that they are impact scars from chains of debris thrown into space by big meteorite impacts on Mars itself.

NOT MARS EJECTA. Calculations find that the grooves on the Martian moon Phobos did not come from chains of debris ejected by impacts on Mars itself. The fact that about 12 "families" of grooves lie parallel, semi-parallel, and intersecting suggests that more than one groove-making episode occurred. (Mars Express image taken from Figure 4 in the paper.)

Writing in Planetary and Space Science, Kenneth Ramsley and James Head (Brown University) say “nix” on this idea. The core of their finding is that the Phobos grooves are too perfectly shaped — too neat and clean — to be the result of ejecta from Martian impacts.

“We strongly suggest that no impact event on Mars produces enough focused material to form grooves as impact chains on Phobos,” the team says. “At the altitude of Phobos, Martian impact debris disperses to a huge volume in the space above Mars. By the time it reaches the altitude of Phobos, the debris is far too thinly distributed to produce more than a few stray impacts on Phobos, if any at all.”

To reach this conclusion, they undertook extensive computer simulations to see how much debris would be ejected from Mars and big the pieces would likely be, how far ejected pieces would travel, how much they would disperse as they flew, and where they would go within the Mars-Phobos system.

“On the basis of our analysis,” they write, “we find that six major predictions of the hypothesis are not consistent with a wide range of Mars ejecta emplacement models and observations.”

These failed predictions include:
• The largest family of grooves can’t be emplaced by any valid trajectory from Mars in its present-day or ancient orbit.
• To make families of parallel grooves over most of Phobos (as is seen), fragments must have nearly identical diameters and be ejected in grid patterns with virtually no dispersion.
• Due to Phobos’ rough and uneven surface, grid patterns of incoming debris would strike the ground more unevenly than is seen, disrupting the grooves’ linearity.
• Grooves are found on the trailing end of Phobos in places where no trajectory from Mars to Phobos is possible.

The researchers also compared the Phobos grooves with chains of known secondary craters on Mercury and the Moon. They found that most Mercurian and lunar secondary craters are as large as the bigger (non-groove) craters on Phobos and far larger than the pits seen in the groove networks.

They also observe that blasting chains of craters across Phobos would also throw secondary plumes into orbit around Mars. Such debris will resettle back onto Phobos over roughly 10,000 years.

As Ramsley and Head explain, “This would substantially add to the effects of space weathering and potentially bury most evidence of the initial groove-forming impacts.